Field of the Invention
This invention generally relates to the processing of materials in a high-temperature operating environment. More specifically this invention relates to the production of crystalline material in high-temperature vacuum furnaces.
Description of Related Art
Although the following discussion relates to the production of crystals in a high-temperature vacuum furnace, it will be apparent that the methods and apparatus disclosed herein are applicable to any high-temperature operating environment used for the production of various products including, but not limited to, the production of various glasses, amorphous materials, multi-crystalline ingots, such as silicon ingots, and thin films.
A process for producing crystals in an environment, such as found in a high-temperature vacuum furnace, can produce any or all of three general categories of unwanted ancillary or secondary reactions that can affect the final crystal quality. They are: (1) high-temperature chemical reactions, (2) a decomposition of unstable compounds and (3) sublimation or vaporization of certain elements.
High-temperature chemical reactions typically involve carbon or refractory metals such as molybdenum and tungsten, which are often used in the construction of high temperature furnaces. If such reactions occur, they can degrade the furnace. Examples of reactions involving graphite-containing furnaces include:C(s)+2Mo(s)→Mo2C  (1)andH2O(g)+C(s)→CO+H2(g)  (2)The carbon monoxide can then react with any refractory metals and thereby form carbides and volatile species.
Molybdenum crucibles also react with Al2O3 at high temperatures in vacuum to form:Al2O3(s)+Mo(s)→MoO2(g)+AlO(g)+Al(g)  (3)and other compounds as well through similar reactions.
Silicon in a silica crucible operating at a melt temperature of 1412° C. and at a pressure below 10 Torr reacts to form gaseous SiO by the reaction:SiO2(s)+Si(s)→2SiO(g)  (4)
At about 1400° C. gaseous SiO reacts with carbon at pressures below 10 Torr to produce highly reactive carbon monoxide (CO) and SiC. That is:2C(s)+SiO(g)→SiC(s)+CO(g)  (5)Silicon carbide can seriously degrade the quality of silicon grown from the melt.
Also, CO reacts with silicon below 10 Torr to form carbon and silicon monoxide according to:CO(s)+Si(s)→C(s)+SiO(g)  (6)Such a reaction can lead to an unwanted deposition of SiO gas onto surfaces in the colder regions of the furnace. In addition, silicon carbide can seriously degrade the quality of silicon grown from the melt.
At elevated operating temperatures certain compounds become unstable and decompose. For example:2MgO(g)→2Mg(s)+O2(g)  (7)whereby free oxygen can react with both the materials used in the furnace itself and with the material being processed in the furnace to form oxides.
As another example, spinel decomposes to magnesium oxide and aluminum oxide according to:MgAl2O4(s)→MgO(g)+Al2O3(s)  (8)where MgO reacts as described above.
Sublimation, or vaporization, occurs when certain elements or compounds are elevated to high temperatures. As known, all metals and some refractory materials are prone to vaporize or sublime at high temperatures. Graphite will sublime at a high temperature into carbon vapor. For example, graphite will sublime into carbon vapor above 2200° C. The carbon vapor can react with a crucible and contaminate the crucible contents.
In crystal manufacturing processes, ancillary reactions, such as reaction (1) above and sublimation, can produce gaseous species. As known, such gas can be captured in the material being processed causing a crystal, for example, to have imperfections, such as inclusion or bubbles, which produces undesirable light scatter.
In high-temperature environments, heating elements are also susceptible to the existence of “hot spots,” which are due to variations in resistivity in the heating element or power source variations, or “cold spots,” which are caused, for example, by leaky insulation. During processing, hot and cold spots can result in non-uniform or non-symmetrical crystal growth. During crystal growth and cooldown, hot spots and/or cold spots produce thermal stress gradients in the crystal that can cause stress defects including dislocations that cause lattice distortion and/or cracking. As known, the existence of fine particles of previous reaction products or of furnace construction materials, such as graphite felt or moisture, that react inside a heat zone can degrade furnace performance and even obscure viewing through viewports.
What is needed is a process and control thereof that minimizes unwanted ancillary reactions and unwanted temperature gradients during processing in high-temperature environments.